1. Field of the Invention
In one aspect this invention relates to torches for plasma arc hard facing. In another aspect, this invention relates to methods of hard surfacing substrates to provide a hard wear resistant coating for service at elevated temperatures, such as service requiring direct contact with engine exhaust gases.
2. Description of the Prior Art
One commonly used plasma arc torch feeds a powder into the center of the torch along its longitudinal axis by means of an inert gas (such as helium). Simultaneously an inert gas such as argon or helium is formed into an ionized plasma by passing the inert gas through an electric arc after which the powdered material and the plasma are combined. The resulting plasma carries and melts the powdered material which exists from the torch and is directed towards an article to be hard surfaced. In general, it is also desirable to have a constant flow of a shield inert gas such as argon or helium about the plasma to prevent oxidation of the molten metal.
In the conventional plasma arc torch there are tortuous turns in the powder's path which allow the powder to plug the torch passages. Consequently, the metal is not fed into the plasma in an even, controlled manner. This design must use the same inert gas for a powder carrier as used to establish the main plasma.
To correct the problem caused by the tortuous path of center feed torches, external feed torches have been developed wherein a powdered material carried by an inert gas is fed into the plasma stream at a point outside the torch. The external feed nozzles minimize plugging by reducing convolutions and tortuous turns in the powder passages especially within the torch. This torch allows the use of a different gas for a powder carrier than that used to establish the main plasma. However, such external feed nozzles have resulted in a new set of problems. One problem is that the resulting nozzle has a large diameter. Large nozzles create problems when hard surfacing shaped work pieces such as engine poppet valves since the nozzle cannot be properly oriented with respect to the work piece. Indeed, some pieces may be impossible to hard face. In addition, a portion of the shaped work piece will often come into contact with the shield gas envelope which results in distortion of the shield and possible degradation of the deposited hard facing. Further, the shaped work piece can also come into physical contact with the torch body which results in shorting the path of the electrical current with subsequent weld defects or damage to the torch.